This invention relates to the field of power transmissions for motor vehicles. More particularly, it pertains to an automatic transmission for an automotive vehicle having an auxiliary gear unit and a primary gear unit. The auxiliary gear unit produces a direct drive ratio and can either underdrive or overdrive its output.
Various techniques have been used to increase the number of gear ratios produced by a multiple speed ratio automatic transmission that employs a simple planetary gearset arranged in series with a compound planetary gearset. For example, U.S. Pat. Nos. 5,755,637 and 5,43,825 disclose an automatic transmission for a motor vehicle that includes a compound planetary gearset coaxial with an input shaft and operating under control of clutches and brakes to establish first to fourth forward drive ratios. A simple planetary gear set coaxial with an output shaft and provides a fifth forward drive ratio by further increasing the pre-established fourth ratio. A chain and sprocket coupler delivers rotary power from the compound planetary gearset to the simple planetary gearset.
There is, however, a need for automatic transmissions to produce an increasing larger number of gear ratios and wider ratio spans. Larger ratio spans improve drive-away performance and fuel economy at the same time. Preferably a six-speed transmission would provide a ratio span of at least 6.0, and a seven-speed transmission would have a ratio span of 8.0 or greater.
It is desired that a transmission having six or seven forward ratios should provide also multiple reverse gear ratios to enhance performance in winter driving conditions. Preferably, a transmission would also provide engine braking at least optionally over a range of forward ratios.